Hepatitis C virus (HCV) is an RNA virus that chronically infects an estimated 120-170 million people worldwide, with up to 20% developing liver diseases such as cirrhosis and hepatocellular carcinoma. The ability of HCV to establish life-long infection attests to its circumvention ofthe host cellular defense. Understanding the relevant cellular pathways for control of HCV and how the virus evades these pathways can lead to better treatments or vaccination strategies. The tripartite motif (TRIM) proteins are a large family of proteins with many functions in eukaryotic cells, including transcriptional control, post-translational modification of proteins, and cell cycle regulation. They have also been shown to be important in the cellular innate antiviral response. We performed a genome-wide siRNA screen to identify host factors that affect HCV genome replication and found 11 TRIM proteins which had a significant effect. Therefore, we hypothesize that these proteins are key mediators of the innate anitviral response to HCV infection. We propose that the TRIMs are exerting their influence directly by interacting with HCV proteins or indirectly, by regulating other pathways that respond to viral infections. Both of these scenarios have strong support in the literature and its likely that out of 11 candidates, we will identify proteins in both groups. To address these questions, we will begin by studying whether TRIM proteins can directly bind to HCV proteins. Further, we will characterize any effect of TRIMs on the early and late stages of the viral life cycle while also trying to identify mutant viruses that can escape the effects ofthe TRIMs. Finding a mutant will allow us to determine how TRIMs affect HCV RNA replication. In addition, we will also investigate whether TRIMs can regulate the cellular pathways important for viral detection and response. This includes the retinoic acid-inducible gene I (RIG-I) pathway that leads to upregulation of type I interferons. We will also investigate any effect of TRIMs on signaling downstream of IFN receptors, on the pathways that lead to upregulation of interferon-stimulated genes.